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Chlamydia in koalas is caused by Chlamydia pecorum, a bacterium that may have spread from livestock introduced from Europe. A similar bacterium, Chlamydia trachomatis, causes chlamydia in humans.

Antibiotics can be used to treat chlamydia in koalas, but they only work in early-stage disease, do not prevent re-infection, and they must be administered daily for at least 30 days in captivity.

Moreover, some infected koalas remain asymptomatic and are overlooked for treatment while they continue to spread the disease.

To address these problems, Peter Timms at the University of the Sunshine Coast in Queensland, Australia, and his colleagues have been developing a single-injection chlamydia vaccine that provides long-lasting protection. Koalas are injected with tiny fragments of C. pecorum bacteria to train their immune systems to fight chlamydia.

The team tested the vaccine on 21 free-ranging koalas in Queensland’s Moreton Bay region. Six had early-stage chlamydia and the other 15 were chlamydia-free.

After six months none of the chlamydia-free koalas had become infected, even though half the koalas in their habitat were carrying the disease. In addition, all six of the individuals that started out with chlamydia had cleared the infection.

Herd immunity

But the vaccine was not as effective at nine months, by which time three of the 21 vaccinated koalas had become infected. This rate of infection was still lower than in the control group – in which five of 21 unvaccinated koalas had contracted chlamydia.

One option is to broaden the coverage of the vaccine by protecting against more strains of C. pecorum. The current formulation only guards against three out of 10 known strains.

Another possibility is to vaccinate more widely. “If you vaccinated the majority of a koala population, it wouldn’t matter so much if the individual protection wasn’t 100 per cent because there would be herd immunity,” Timms says.

His team is about to trial this latter strategy in a group of around 50 wild koalas in the suburb of Petrie in Queensland. Roads and houses border their habitat, meaning they are separated from other koalas. All 50 koalas will be vaccinated to determine whether the resulting herd immunity reduces the overall disease burden of the population.

“You’ll never be able to get rid of chlamydia completely – same as you can never get rid of the flu – but we think the vaccine could at least turn koala populations around so they’re going up instead of down,” says Timms.

The team plans to apply for government approval for the vaccine within the next 12 months.

Ken Beagley at the Queensland University of Technology, who co-developed the koala chlamydia vaccine, is now using the same principles to develop a human version containing fragments of C. trachomatis bacteria instead of C. pecorum.

Using a mouse form of the human vaccine, his team has shown that it slows the spread of chlamydia when both male and female mice are vaccinated. They hope to conduct a clinical trial of the human vaccine in the next five years.